In order to assess the individual and synergic cytotoxicities of harmful components in cigarette smoke, bronchus epithelia, BEASe-2B, were used as target cells and exposed to the five harmful components in cigarette smoke, NNK, NH₃, B[a]P, phenol and crotonaldehyde, individually and in pairs, the survival rates of the cells after 24-hour exposure were tested by MTT assay, average LC₅₀ of the cells was calculated, and the synergic cytotoxicities were evaluated with isobolograms. The results showed that: 1) LC₅₀ of NNK, NH₃, B[a]P, phenol, crotonaldehyde, NNK+NH₃, NNK+B[a]P, NNK+phenol, NNK+crotonaldehyde, NH₃+B[a]P, NH₃+phenol, NH₃+crotonaldehyde, B[a]P+phenol, B[a]P+crotonaldehyde and phenol+crotonaldehyde was 87.2, 395.5, 5.4, 61.9 , 6.9, 46.2, 10.9, 14.1, 30.4, 76.4, 86.1, 39.0, 6.1, 1.1 and 22.1 mg/L, respectively. It suggested that there existed potent synergism between NNK and NH₃, NNK and B[a]P, NNK and phenol, B[a]P and phenol, B[a]P and crotonaldehyde, NH₃ and crotonaldehyde, while no synergism between NNK and crotonaldehyde, B[a]P and NH₃, phenol and NH₃, phenol and crotonaldehyde in cytotoxicity.